Field of the Invention
The present invention relates to a switching power supply apparatus and an image forming apparatus that use an active clamp method for an insulated type converter using a flyback transformer.
Description of the Related Art
An active clamping power supply apparatus using a flyback transformer is known as one of the configurations of a power supply apparatus having a high power efficiency for both light load and heavy load. In order to obtain a higher power efficiency, Japanese Patent Application Laid-Open No. 2009-100554 proposes a power supply apparatus configured to achieve a high power efficiency both at the time of light load and at the time of heavy load by switching the capacitance of a resonance capacitor connected in parallel with a switching element, according to the magnitude of load to which power is supplied. Note that the power efficiency (also called the power conversion efficiency) is represented by the ratio of the power output by the power supply apparatus to the power supplied to the power supply apparatus.
As described above, in the power supply apparatus, the capacitance of a resonance capacitor is switched according to the load to which the power is supplied. Especially, in a partial resonance type power supply apparatus, such as the active clamping power supply apparatus using a flyback transformer, in order to achieve a stable switching operation and a further improvement of the power efficiency, the timing for switching the capacitance of a resonance capacitor has been an important issue.